Polymeric membranes, such as cured sheets of ethylene-propylene-diene copolymer rubber (EPDM) or extruded sheet of thermoplastic olefins (TPO), are often used in the construction industry to cover flat or low-sloped roofs. These membranes, which may also be referred to as panels, are typically delivered to a construction site in a bundled roll, transferred to the roof, and then unrolled and positioned. The sheets are then affixed to the building structure by employing varying techniques such as mechanical fastening, ballasting, and/or adhesively adhering the membrane to the roof. The roof substrate to which the membrane is secured may include a variety of materials depending on the situation. For example, the surface may be a concrete, metal, or wood deck, it may include insulation or recover board, and/or it may include an existing membrane.
In addition to securing the membrane to the roof—which mode of attachment primary seeks to prevent wind uplift—the individual membrane panels, together with flashing and other accessories, are positioned and adjoined to achieve a waterproof barrier on the roof. Typically, the edges of adjoining panels are overlapped, and these overlapping portions are adjoined to one another through a number of methods depending upon the membrane materials and exterior conditions. One approach involves providing adhesives or adhesive tapes between the overlapping portions, thereby creating a water-resistant seal.
Thus, there are two modes of membrane attachment that are used in conjunction. The first seeks to anchor the membrane to the roof, while the second seeks to create a water-impervious barrier by attaching individual adjacent membrane panels to each other or to flashing. Inasmuch as these modes of membrane attachment seek entirely different goals, the mechanisms by which they operate are likewise distinct.
Adhesive attachment is typically employed to form adhered roofing systems. The membrane may be adhered to the roof substrate substantially across the entire planar surface of the membrane to form fully-adhered systems. In other words, a majority, if not all, of the membrane panel is secured to the roof substrate as opposed to mechanical attachment methods which can only achieve direct attachment in those locations where a mechanical fastener actually affixes the membrane. Fully-adhered roofing systems are advantageously installed where maximum wind uplift prevention is desired. Also, fully-adhered systems are desirable in re-roofing situations, especially where the new membrane is placed over an existing membrane (a technique that is commonly referred to as re-skinning).
Several techniques are employed to prepare fully-adhered roofing systems. One technique includes the use of a fleece-backed EPDM membrane that is secured to the substrate by using a low-rise polyurethane foam adhesive that is sprayed over the substrate. Once the adhesive polyurethane foam is applied, the fleece-backed membrane is applied to the adhesive layer, which attaches itself to the fleece backing. Alternatively, nitrile-based bond adhesives can be applied to the substrate and the fleece-backed EPDM membrane can be secured thereto. Because these systems require fleece-backed membranes, they are expensive and suffer from manufacturing inefficiencies relating to the need to secure the fleece to the membrane.
Other techniques employ conventional EPDM membrane sheet, which is not modified with a fleece backing. In these situations, it is common to employ a contact bonding method whereby technicians coat both the membrane and the substrate that receives the membrane with an adhesive. The adhesive is then typically allowed to at least partially set to, among other things, build some wet green strength. The membrane is then mated with the substrate via the partially-set adhesive. Because the volatile components (e.g. solvent) of the adhesives are “flashed off” prior to mating, good, early (green) bond strength can advantageously be developed.
One technique employs a water-borne bond adhesive that is applied to the substrate and then the EPDM membrane can be applied to the adhesive layer. While this attachment technique has proven useful, the use is generally limited to ambient weather conditions (e.g. greater than 40° C.) and/or in conjunction with porous substrates that absorb water thereby allowing the adhesive to dry or cure without blistering the membrane.
In other situations, solvent-based adhesives are employed, such as polychloroprene-based bond adhesives. While the use of known solvent-based adhesives has proven versatile to the extent that the substrate need not be porous and cold-weather application is feasible, the technique requires application of the adhesive to both the substrate and the membrane, followed by a time delay to allow the solvent to flash off, and then a mating of the two adhesive surfaces (i.e., the adhesive coated membrane is mated to the adhesive coated substrate).
In yet other situations, 100% solids bond adhesives are employed. For example, U.S. Pat. No. 7,767,308 teaches a moisture-curable bond adhesive that includes a polymer or a combination of polymers having silicon-containing hydrolyzable terminal groups, a phenolic resin, and a non-polymeric silicon-containing hydrolyzable compound. While these bond adhesives are touted for being free of volatile organic compounds (VOCs), safe for chronic exposure, and non-flammable, and yet provide a high initial peel strength and/or high peel strength upon being fully cured between a roof substrate and a rubber membrane, it would nonetheless be desirable to formulate a bond adhesive that does not include a phenolic resin.
While both solvent-based and water-based adhesives may be used as contact adhesives, solvent-based bonding adhesives offer advantages. For example, the flash-off period, which is the time required to allow solvent evaporation prior to mating, can be between 5 and 40 minutes, and is less susceptible to environmental conditions, such as temperature, than water-based adhesive systems. Solvent-based systems, on the other hand, can be problematic. For example, the solvent employed in the system can cause membrane swelling and/or blistering. It is believed swelling and blistering results from solvent compatibility with the membrane and/or a component of the membrane. Other problems can include blushing, which is the formation of condensation on the surface of the film formed upon application of the adhesive to the membrane. Blushing can have a deleterious impact on the bond strength and/or quality of the bond formed by the adhesive and is therefore not desirable.
The evaporation of solvents can be problematic, especially as the desire to minimize release of volatile organic compounds increases. Thus, both water-borne and solvent-borne systems known in the art today have limitations, and there is therefore a desire for a bond adhesive that overcomes these advantages.